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| United States Patent |
5,010,054
|
|
Johansen
, et al.
|
April 23, 1991
|
Method for the preparation of superconducting products
Abstract
Superconducting products on the basis of crystalline superconducting
compounds having the general formula (Me.sup.1).sub.p (Me.sup.2).sub.q
Cu.sub.3 O.sub.x F.sup.Y, wherein Me.sup.1 is selected from Y, Sc and the
lanthanides, Me.sup.2 is Ba and/or Sr, p is 0.8-2.5, q is 0.8-3, x is
5-8.5 and y is 0-6, are prepared by reacting oxides of the metals in the
molten phase, the Me.sup.2 oxides at least in part being used as Me.sup.2
peroxide and Me.sup.1 oxide and/or Cu oxide optionally being partly
replaced by fluorides. The reaction is carried out under an oxygen-free
atmosphere in a closed reaction vessel having deformable walls, and during
the reaction the vessel is placed in an autoclave under an isostatic
pressure of 800 to 3000 bar and at a temperature of 800.degree. to
1400.degree. C., after which cooling is carried out without opening of the
vessel. During, between or after the heating and cooling the vessel with
its contents is shaped to the desired intermediate product or final
product having superconducting properties. Very suitably the reaction
vessel may be a copper tube which during, between or after the heating and
cooling is drawn to form a superconductor cable.
| Inventors:
|
Johansen; Keld (Frederikssund, DK);
Pedersen; Erik (Horsholm, DK);
Topsoe; Haldor F. A. (Vedb k, DK)
|
| Assignee:
|
Haldor Topsoe A/S (DK)
|
| Appl. No.:
|
216895 |
| Filed:
|
July 8, 1988 |
Foreign Application Priority Data
| Current U.S. Class: |
505/432; 505/491; 505/704; 505/739; 505/740 |
| Intern'l Class: |
H01B 012/00; H01L 039/00 |
| Field of Search: |
505/1,704,739,740
264/65
|
References Cited
U.S. Patent Documents
| 4518546 | May., 1985 | Greskovich et al. | 264/65.
|
| Foreign Patent Documents |
| 60-165005 | Aug., 1985 | JP.
| |
| 61-223170 | Oct., 1986 | JP.
| |
| 214264 | Jul., 1967 | SE.
| |
Other References
Koa, "Synthesis of yttrium Larium Copper Oxide Superconductors from Y.sub.2
O.sub.3, B.sub.0 O.sub.2, Cu.sub.2 O: the Optional Oxygen Treatment",
Mater. Lett., 6(3), 53-7, early 1987.
Larbalestier et al., "Microstructural and Electromagnetic Characterization
of La.sub.2-x Sr.sub.y CuO.sub.4 ", Mar. 18, 1987, Cryogenics, Aug. 1987.
Robinson et al., "Sinter-Forged YBa.sub.2 Cu.sub.8 O.sub.7-8 ", Adv. Cer.
Matl., vol. 2, 3D, 380-7, Jul. 1987.
|
Primary Examiner: Dixon, Jr.; William R.
Assistant Examiner: Bonner; Melissa
Attorney, Agent or Firm: Ostrolenk, Faber, Gerb & Soffen
Claims
What is claimed is:
1. In a method for the preparation of a superconducting product on the
basis of superconducting compounds having the general formula (Me.sup.1)
(Me.sup.2 ).sub.q Cu.sub.3 O.sub.x, where Me.sup.1 is at least one metal
selected from the group consisting of yttrium, scandium and the
lanthanides, Me.sup.2 is at least one of the metals barium and strontium,
p is a number from 0.8 to 2.5, q is a number from 0.8 to 3 and x is a
number from 6 to 8.5, in which process oxides of Me.sup.1, Me.sup.2 and Cu
are reacted with each other at an elevated temperature,
wherein the improvement comprises
(i) employing an oxide of Me.sup.2 at least partly as the Me.sup.2
peroxide,
(ii) placing the oxides under an oxygen-free atmosphere in a reaction
vessel having deformable walls,
(iii) closing the vessel hermetically and placing it with its contents in
an autoclave suitable for hot isostatic pressing, gradually subjecting it
to a pressure of 800 to 3000 bar, heating it to a temperature in the range
of 800 to 1400.degree. C. and then cooling it without opening it, and
(iv) shaping the vessel with its contents at any time from the beginning of
the heating procedure to the end of the cooling in order to form a desired
product having superconducting properties.
2. A method according to claim 1, wherein gases are evacuated from the
vessel before it is closed hermetically.
3. A method according to claim 1, wherein the cooling takes place
spontaneously.
4. A method according to claim 3, wherein after the completion of the
heating, the heat supply to the autoclave is discontinued to allow
spontaneous cooling in the autoclave of the vessel with its contents.
5. A method according to claim 1, wherein the reaction vessel having
deformable walls is a copper tube which is heated to a temperature of
between 800.degree. C. and 1100.degree. C. and drawn to form a
superconductor cable.
6. In a method for the preparation of a superconducting product on the
basis of superconducting compounds having the general formula
(Me.sup.1).sub.p (Me.sup.2).sub.q Cu.sub.3 O.sub.x F.sub.y, where Me.sup.1
is at least one metal selected from the group consisting of yttrium,
scandium and the lanthanides, Me.sup.2 is at least one of the metals
barium and strontium, p is a number from 0.8 to 2.5, q is a number from
0.8 to 3, x is a number from 6 to 8.5 and y is a number greater than 0 to
6, in which process oxides and fluorides of Me.sup.1, Me.sup.2 and Cu are
reacted with each other at an elevated temperature,
wherein the improvement comprises
(i) employing an oxide of Me.sup.2 at least partly as the Me.sup.2
peroxide,
(ii) placing the oxides together with at least one fluoride selected from
the group consisting of fluorides of Me.sup.1 and Cu under an oxygen-free
atmosphere in a reaction vessel having deformable walls,
(iii) closing the vessel hermetically and placing it with its contents in
an autoclave suitable for hot isostatic pressing, gradually subjecting it
to a pressure of 800 to 3000 bar, heating it to a temperature in the range
of 800 to 1400.degree. C. and then cooling it without opening it, and
(iv) shaping the vessel with its contents at any time from the beginning of
the heating procedure to the end of the cooling in order to form a desired
product having superconducting properties.
7. A method according to claim 6, wherein gases are evacuated from the
vessel before it is closed hermetically.
8. A method according to claim 6, wherein the cooling takes place
spontaneously.
9. A method according to claim 8, wherein after the completion of the
heating, the heat supply to the autoclave is discontinued to allow
spontaneous cooling in the autoclave of the vessel with its contents.
10. A method according to claim 6, wherein the reaction vessel having
deformable walls is a copper tube which is heated to a temperature of
between 800.degree. C. and 1100.degree. C. and drawn to form a
superconductor cable.
Description
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a method for the preparation of
superconducting products on the basis of superconducting compounds having
the general formula (Me.sup.1).sub.p (Me.sup.2 ).sub.q Cu.sub.3 O.sub.x
F.sub.y, where Me.sup.1 is one or more metals selected from yttrium,
scandium and the rare earth metals, Me.sup.2 is barium and/or strontium, p
a number 0.8-2.5, q a number 0.8-3, x a number 5-8.5 and y a number 0-6,
in which process oxides and optionally fluorides of Me.sup.1, Me.sup.2 and
Cu are reacted with each other at an elevated temperature.
It is known that metal-oxide ceramic compounds of this kind, which are
frequently called Y--Ba--Cu--O and in which the metals are present in the
oxidic form, possess superconducting properties at a substantially higher
temperature than that known for classic superconductors (up to about 23
K), frequently at a little above the boiling point of liquid nitrogen at
atmospheric pressure. It is also known that a partial replacement of
oxygen by fluorine may improve the superconducting properties.
Although the metals in compounds of this kind typically are yttrium, barium
and copper, yttrium may be entirely or partly replaced by scandium and/or
one or more lanthanides, and barium entirely or partly by strontium. In
the materials of the kind so far known, copper is compulsory; it is known
that the superconducting properties are improved concurrently with the
degree to which copper is present at a higher oxidation state than 2,
preferably predominantly or entirely in oxidation state 3. Known materials
of the kind in question have, for example, the composition YBa.sub.2
Cu.sub.3 O.sub.z, where z is a number 6 to 8, or La.sub.1.85 Ba.sub.0.15
CuO.sub.4.
Surveys of superconducting substances of the kind in question are, i.a., an
article by Ron Dagani in Chemical & Engineering News, May 11, 1987, p.p.
7-16, and by Ib Johannsen and Thomas Bj rnholm in Dansk Kemi 5, May,1987,
p.p. 172-175. There is a bibliography in the latter.
It is known that such compounds can be prepared by a solid phase reaction
between suitable compounds, e.g. oxides, carbonates or oxalates, in powder
form under an oxygen-containing atmosphere; that the reaction must be
conducted under a controlled temperature profile; and that the composition
of the surrounding atmosphere is important, notably the partial pressure
of oxygen. Hitherto one has always used barium oxide, BaO, and possibly
strontium oxide, SrO, as the source of Me.sup.2 and conducted the reaction
in an oxygen-containing atmosphere, frequently in pure oxygen.
It may hereby be difficult to control the temperature profile and the
oxygen pressure of the surrounding atmosphere so closely as to obtain a
well-defined product, especially a compound in which Cu is present in a
state of oxidation above 2. It might be important to provide another
source of oxygen than elemental oxygen from the atmosphere or in an
industrailly commonly available pure form.
The known processes for the preparation of Y--Ba--Cu--O compounds have the
well-known drawbacks of solid phase reactions, including a slow reaction,
difficulties in obtaining the needful contact between the reactants,
difficulties in obtaining uniform products having high contents of desired
phases and in obtaining a desired crystalline form of desired purity and
dimensions, notably in obtaining perfect crystals, monocrystals.
The superconducting crystals prepared by the known processes must
frequently be employed not as they are, but must be worked up and shaped
to intermediate products or final products, e.g. superconductor cables.
Such working up frequently involves complicated and difficult methods.
DISCLOSURE OF THE INVENTION
It has now been surprisingly found that an overall solution of these
problems can be obtained if according to the invention the Me.sup.2 oxide
at least partly is employed as the Me.sup.2 peroxide and the oxides,
possibly accompanied by fluorides of Me.sup.1 and/or Cu, are placed under
an oxygen-free atmosphere in a reaction vessel having deformable walls,
which vessel is thereafter hermetically closed and placed in an autoclave
suitable for hot isostatic pressing (HIP), gradually subjected to a
pressure of 800 to 3000 bar and heated to a temperature in the range of
800 to 1400.degree. C. and then cooled without being opened, the vessel
with its contents being shaped during, between or after the heating and
cooling to form a desired intermediate product or final product having
superconducting properties.
When using barium peroxide and/or strontium peroxide as the source of Ba
and/or Sr it is obtained that these compounds can also function as oxygen
sources and that the reaction can be carried out without any outer oxygen
supply. By adjusting the proportion of Me.sup.2 peroxide relative to the
Me.sup.2 oxide it is possible to a considerable degree to regulate the
degree to which Cu will be present in the final compound in an oxidation
state above 2; by using only Me.sup.2 peroxides it is possible at least to
approach the theoretical maximum value of x of 8.5; if Me.sup.1 and Cu are
employed partly as fluorides this maximum value of x must of course be
decreased corresponding to the value of y.
Moreover, barium and strontium peroxide as components of the starting
material have the advantage of low melting temperatures, 450.degree. C.
and 215.degree. C., respectively, whereas the monoxides melt at
1923.degree. C. and 2430.degree. C., respectively. It is thus achieved
that the reaction between the starting materials partly can take place in
the molten phase, which ensures a better contact between the reactants
than that obtained in the known solid phase reactions.
Finally the conducting of the reaction in a deformable vessel ensures that
the latter at any time, together with its contents of Y--Ba--Cu--O
compound or starting material therefor or any intermediate step of its
preparation may be shaped to form an intermediate product or end product
having a desired outer shape and being suitable for commercial purposes or
for scientific experiments.
Since the liquid phase of the Me.sup.2 peroxide allows a high rate of
reaction, it involves good possibilities of obtaining perfect
monocrystals. When the reaction is carried out under HIP conditions as
stated, a high density of matter can be obtained, which may be considered
a condition for obtaining a high current density in the finished product.
By suitable selection of reaction temperature and temperature gradients,
notably during the heating but to a certain degree even during the
cooling, it is possible to promote the possibilities of obtaining large
monocrystals having a high density of matter.
During the heating the starting materials to some degree are transformed
into the gaseous phase. In order to avoid a too high gas pressure in the
vessel and to limit the amount of gas in the final product, it may
according to the invention be appropriate to evacuate the vessel to a
considerable degree before it is hermetically closed.
As mentioned the heating is carried out gradually, and according to the
invention it is expedient that the cooling takes place gradually and
slowly, e.g. by taking place spontaneously thereby that the heat supply to
the autoclave is discontinued and the product removed only when cooling
has taken place quite or almost to ambient temperature.
According to the invention the deformable reactor may advantageously be a
metal tube, e.g. of copper or a stainless alloy (of a high purity suitable
for the purpose) which during, between or after the heating and cooling is
drawn to form a superconductor cable.
The autoclave employed must be arranged so as to allow a close temperature
regulation in the range from 0.degree. C. to 1100.degree. C., preferably
up to 1400.degree. C. The temperature rise should be controlled carefully,
especially in the range from somewhat below the melting point of the
Me.sup.2 peroxide to the final temperature which in the case of using
barium peroxide conveniently is around 950.degree. C.
The method according to the invention is to be illustrated by an example in
the following.
EXAMPLE
In a mortar 1.13 g (0.005 mol) of Y.sub.2 O.sub.3, 3.39 g (0.020 mol) of
BaO.sub.2 and 2.39 g of (0.030 mol) CuO were comminuted thoroughly into
powders and were placed in a tube of a supple alloyed steel having a wall
thickness of 0.3 mm and an inner diameter of 3.2 mm. After compacting in
known manner the tube was closed and placed in an autoclave suitable for
hot isostatic pressing and adapted to temperature control within the range
of 0 to 1400.degree. C. The tube with its contents was subjected in the
autoclave to a pressure of 2600 bar, generated by the compression of
nitrogen, and then gradually heated to 915.degree. C. during which the
heating from 300.degree. C. to 915.degree. C. took place at a rate of
about 143.degree. C. per hour. The temperature of 915.degree. C. and the
pressure of 2600 bar were maintained for 3 hours, after which the tube
with contents was cooled spontaneously by discontinuing the heat supply to
the autoclave.
The product formed has superconducting properties (a clear Meissner effect)
and permits a current density of above 1000 A/cm.sup.2.
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